This bug still happens with patch 1.9. It's rare but it's annoying because you only notice it when it's too late. It seems to be connected to cell loads just like other settlement bugs that cause your settlement data to become corrupted (incorrect number of food, water, power, beds, defense). It may also be related to the infamous cell reset bug which happens in certain locations. I just had a provisioner sentry bot revert to basic protectron. It was set up between Greentop Nursery and The Slog. On that route it passes Breakheart Banks, a supermutant farm that resets instantly if you move within a certain distance to it. Fortunately I was able to fix it with a slightly earlier savegame which I did before leaving the Slog. I pulled the sentry bot to the Slog by using the robot workbench, teleported over to Greentop Nursery, did some management, pulled the sentry to Greentop Nursery, teleported to Outpost Zimonja to manage stuff over there and teleported back to the Slog just like I did before. When I pulled the bot to the Slog it was still a sentry bot instead of the protectron that I met on the road to Saugus on the previous attempt. Quite a relief because sentry bots are super expensive in terms of rare materials... On a previous playthrough (with patch 1.8) I had similar issues with a provisioner robobrain between Mechanist's Lair and County Crossing. Fast traveling to County Crossing had a good chance to revert that bot to a protectron. Another provisioner bot that experienced the issue was one between Dalton Farm and National Park Visitor's Center in Far Harbor.